The present invention relates in general to a semiconductor memory device, and in more particular, to a high-density semiconductor memory device having memory cells using capacitance coupling.
The high integration for producing a semiconductor memory device is one of the important technical subjects which are constantly being pursued to increase the capacity of the memory. In the existing circumstances, a dynamic RAM (random access memory) is known as one of the semiconductor memory devices with the highest integration. The dynamic RAM includes a plurality of cells each of which consists of a single MOSFET and a single MOS capacitor. However, in the future, it is obvious that a demand for the highly integrated semiconductor memory device as a VLSI memory of the next generation will further increase. It cannot be expected that this kind of demand will be completely or sufficiently satisfied by the semiconductor memory device having the existing structure. This is because in the semiconductor memory device to which the conventional memory structure has been applied as is, the area of each memory cell is decreased in order to realize high integration; the capacitance of the capacitor included in one cell is decreased in its dependence; the amount of charge storage per cell is reduced; and as a consequence, data cannot be sufficiently stored. In other words, according to the technical approach such as a simple reduction in the area of the existing memory cell structure, the degree of integration of the semiconductor memory device is limited, and so it is impossible to attain a highly integrated semiconductor memory device that is satisfactory.
In place of the conventional semiconductor memory device of the charge storage type, a new semiconductor memory device having memory cells of the current readout type has been recently reported. A typical semiconductor memory device of this kind is fully described in K. Terada et al, "A New VLSI Memory Cell Using Capacitance Coupling," IDEM 82 Tech. Dig., 1982, pp 624 -627. However, according to the semiconductor memory device having memory cells using capacitance coupling, in the case of rewriting data in an arbitrary memory cell, a discharge may occur in the other memory cells connected to the common word line (which is also used for the memory cell). This causes the drawback that the contents of the other memory cells can be erased. Therefore, in a semiconductor memory device of the current readout type, in the case of rewriting data in the arbitrary memory cells, it is necessary to perform the troublesome operations such that the data stored in the other memory cells connected to the common word line (which is also used for the arbitary memory cell) be once read out and then transferred to the external memory which has been preliminarily prepared, before they are again rewritten into the corresponding memory cells. This procedure adversely affects (1) the high-speed operation and (2) the conservation of electric power consumed by the semiconductor memory device.